This invention relates to screening apparatus as encountered in the pulp and paper industry. The screening apparatus separates rejectable material from acceptable material when both are in a liquid suspension.
In the papermaking process, a screening apparatus is typically used to separate foreign material from a fluid pulp. For example, a screening apparatus for recycling paper materials, is often fed material having non-recyclable materials, (e.g. plastics, foams, self-stick adhesives) which are all required to be separated from the recyclable paper.
A typical screening apparatus includes a housing within which a screen basket is mounted around a concentrically positioned rotor assembly. The screen basket is usually made from a relatively thin metal plate material having openings sized to screen the non-recyclable materials. When mounted in a screening apparatus, the screen basket provides a barrier between a screening chamber and an accept chamber. Fluid pulp is introduced to the screening chamber by way of a feed inlet which is typically at the screening chamber. Material unable to pass through the screen basket flows to an end of the screening chamber away from the feed inlet and is removed through a reject outlet. Acceptable material passing through the screen basket to the accept chamber moves on to the accept outlet for further processing.
The rotor assembly generally includes pulse elements such as, hydrofoils or a drum cylinder having a contoured surface, mounted on a rotating shaft and in close proximity to the screen basket to sweep past the openings of the screen basket. The pulse elements may be positioned on either the feed or accept side of the screen basket. The rotating pulse elements generate hydrodynamic pulses in the radial direction with enough force and frequency to continuously remove any fiber plugs that occur in the screen basket openings.
In operation, fluid pulp introduced under pressure to the feed inlet circulates along the outer radial portion of the screen apparatus housing. A spinning action is imparted to the circulating fluid pulp by the rotational motion of the rotor assembly such that materials with low specific gravity accumulate along the axis of rotation of the rotor assembly and materials of higher specific gravity move to outer radial portions of the housing. One type of screening apparatus includes an air bleed placed along the axis of rotation and within the feed chamber so that air and light materials amassed at the axis of rotation can be removed.